Liquid crystal projection display

ABSTRACT

The liquid crystal is written with a laser beam. The laser beam generated by a laser source is applied through an optical control unit such as galvanometer mirrors to the liquid crystal. The intensity of the laser beam applied to the liquid crystal is controlled by a laser beam intensity varying unit. A writing control unit controls the mechanical displacement caused by the optical axis control unit and controls the laser beam intensity varying unit. The laser beam energy applied to the liquid crystal can be maintained constant, thus enabling a constant written line width even during acceleration and deceleration of the optical axis control unit.

This is a continuation of application Ser. No. 223,709, filed July 22,1988 U.S. Pat. No. 4,810,064, which is a Continuation of applicationSer. No. 861,007, filed May 8, 1986, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal projection displaywherein a liquid crystal is thermally written with a laser beam.

2. Description of the Related Art

A demand for a large screen display has recently become great in controlapplications, particularly for such a display capable of displayinggraphic patterns with high resolution. The liquid crystal projectiondisplay which uses a laser beam for thermal writing to the liquidcrystal is a promising candidate meeting such requirements.

In the liquid crystal projection display, writing to the liquid crystalis achieved by deflecting a laser beam generated by a laser source withoptical axis control means such as galvanometer mirrors and irradiatingit onto the liquid crystal. Deflection of the laser beam is performed inthe two directions of X axis and Y axis. As optical axis control means,a galvanometer type deflector or a rotary polygon mirror type deflectoris employed which effects laser beam deflection by means of itsmechanical displacement. In this case, however, the speed ofdisplacement varies at the start and end of the mechanical displacement,and hence at the start and end of the deflection of the optical axis. Asa result, there arises a problem that the width of a written linechanges.

To solve this problem, it is known in the art that a laser beam forthermal writing is raster-scanned wider than the width of the liquidcrystal to ensure a constant speed over the surface of the liquidcrystal itself and a constant width of written lines. This technology isdescribed in "Proceeding of the S.I.D.", Vol. 19/1, 1978, at pp.1 to 7.

With this technology, it is necessary for the optical axis controller tobe mechanically displaced even to the outside of the writing surface ofthe liquid crystal, resulting in a disadvantage that the writing timebecomes long.

SUMMARY OF THE INVENTION

In view of the above problem, it is an object of the present inventionto provide a liquid crystal projection display capable ofthermal-writing a constant width line in short time using a laser beam.

The characteristic feature of the present invention resides in that theintensity of a laser beam irradiated on the liquid crystal is changedwith the deflection speed of the optical axis. In other words, thepresent invention is characterized in that energy of the laser beamapplied to the liquid crystal is always maintained constant irrespectiveof the deflection speed of the optical axis.

The above and other objects and features will become apparent from thefollowing description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the construction of the liquidcrystal projection display according to an embodiment of the presentinvention;

FIG. 2 schematically shows the structure of the acousto-optic modulator;

FIG. 3 is a detailed block diagram showing the write control circuit andthe galvanometer mirrors;

FIGS. 4(a-b) and 5(a-b) are graphs for explaining the performance of thepresent invention;

FIG. 6 is a graph illustrating the performance of the speed/lightquantity conversion circuit;

FIG. 7 is a graph showing the written line width against the opticalaxis deflection speed;

FIG. 8 is a graph showing the laser beam intensity against the writtenline width;

FIG. 9 is a graph showing the laser beam intensity against the opticalaxis deflection speed;

FIG. 10 is a graph showing the written line width against the positionof the laser beam under acceleration;

FIG. 11 is a coordinate plane for explaining writing to the liquidcrystal; and

FIG. 12 shows timing charts for explaining the speed limit values.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The embodiment of the present invention is shown in FIG. 1. In thefigure, a laser beam generated by a laser source 1 passes a modulator 2and is reflected by a mirror 3 to change its optical path. The modulator2 changes the laser beam intensity as will be described later. The beamdiameter of the laser beam reflected by the mirror 3 is adjusted to apredetermined size at an expandor 4 and applied to galvanometer mirrors5X and 5Y. The galvanometer mirror 5X deflects the laser beam in the Xdirection, while the galvanometer mirror 5Y deflects the laser beam inthe Y direction. The galvanometer mirrors 5X and 5Y continuously changetheir physical positions respectively based on an X axis positioncontrol signal and a Y axis position control signal supplied from awriting control circuit 7 to change the direction of the optical axis ofthe laser beam. The writing control circuit 7 may usually be amicroprocessor. The laser beam with its optical axis deflected by thegalvanometer mirrors 5X and 5Y is collected by a scan lens 6 and appliedto a smectic liquid crystal 10. A screen controller 8 is inputted withposition information from an unrepresented host computer to obtain an Xaxis position instruction signal and a Y axis position instructionsignal which, in turn, are inputted to a writing control circuit 7. Thecontroller 8 is supplied from the host computer, besides the positioninformation, with display information either to write or erase, patterninformation of a pattern to be written or erased, and so on. The screencontroller 8 also controls the amplitude of an AC voltage applied to theliquid crystal 10 and its timing. Application of an AC voltage to theliquid crystal 10 is for performing either a full erase or a selectiveerase. The laser source 1, modulator 2, mirror 3, expandor 4,galvanometer mirrors 5X and 5Y, scan lens 6, and writing control circuit7 constitute a writing control system 9. Light from a light source 11such as a xenon lamp is made parallel by a lens 12 and irradiated to ahalf-mirror 13. Half of the light quantity of the parallel light appliedto the half-mirror 13 is transmitted therethrough, while the other halfis applied to the liquid crystal 10. The light applied to the liquidcrystal 10 is reflected from the area other than written lines. Thereflected light is projected on a screen 16 via the half-mirror 13 and aprojector lens 14. The light source 11, lens 12, half-mirror 13 andprojection lens 14 constitute a projection system 15.

FIG. 2 shows an example of the modulator 2. Modulators are divided intothose utilizing the electrooptical effect and those utilizing theacousto-optic effect. The modulator shown in FIG. 2 utilizes theacousto-optic effect.

Referring to FIG. 2, an incident laser beam I_(i) generated by the lasersource 1 is collected by a condenser 24 to be applied to a glass member22 to which a piezoelectric element 23 is fixed. The piezoelectricelement is driven by a piezoelectric element driver circuit 26. Uponvibration of the piezoelectric element 23, ultrasonic waves arepropagated into the glass member 22. The incident laser beam I_(i) isthen split by a diffraction grating 22a into a first order beam I₁ and aO order beam I₀. The first order beam I₁ is made parallel by a lens 24and applied to the mirror 3, while the O order beam I₀ is intercepted bya shutter 25.

A detailed block diagram of the writing control circuit 7 and thegalvanometer mirrors 5X and 5Y is shown in FIG. 3. In the figure, theelements associated with the X axis are represented by suffix x, whilethe elements associated with the Y axis are represented by suffix y.

Referring to FIG. 3, a position control circuit 31X is inputted with anX axis position instruction signal P_(xp) supplied from the controller 8and an X axis position signal P_(xf) detected with a position detector53X detecting the position of the X axis mirror, and outputs a speedinstruction signal v_(xp) corresponding to a position deviation. Thespeed instruction signal v_(xp) is applied to a speed control circuit33X via a speed limit circuit 32X. An X axis speed limit value v_(xL) isset at the speed limit circuit 32X by the controller 8. The X axis speedlimit value v_(xL) is determined by the positions of the writing startand end positions on an X-Y coordinate plane. The detailed descriptionof the X axis speed limit value v_(xL) will be given later. The speedcontroller 33X is inputted with the speed instruction signal v_(xp) anda speed detecting signal v_(xf) detected by the speed detector 52X, andoutputs a current instruction signal i_(xp) corresponding to a speeddeviation to apply it to a current control circuit 34X. The currentcontrol circuit 34X actuates a mirror driver circuit 51X based on thecurrent instruction signal i_(xp) and drives a mirror 54X. A currenti_(xf) flowing through the mirror driver circuit 51X is fed back to thecurrent control circuit 34X. The deflection speed and position of themirror 54X are detected by the speed detector 52X and a positiondetector 53X both mechanically coupled to the mirror 54X. Similarly, thegalvanometer mirror 5Y for the Y axis is driven based on a Y axisposition instruction signal P_(yp) by means of a position controlcircuit 31Y, speed limit circuit 32Y, speed control circuit 33Y andcurrent control circuit 34Y. An optical axis deflection speedcalculating circuit 35 is inputted with an X axis deflection speedsignal v_(xf) and a Y axis deflection speed signal v_(yf) respectivelydetected by the speed detectors 52X and 52Y to obtain an optical axisdeflection speed v_(lo) which, in turn, is inputted to a speed/lightintensity conversion circuit 36. The speed/light intensity conversioncircuit 36 obtains the intensity of the laser beam to be applied to theliquid crystal based on the optical axis deflection speed v_(lo), andinputs a light intensity control signal l_(c) to the modulator 2 via aswitching circuit 37. An optical axis deflection speed instructioncalculating circuit 38 is inputted with an X axis speed instructionsignal v_(xp) and an Y axis speed instruction signal v_(yp) respectivelyoutputted from the speed limit circuits 32X and 32Y, and obtains anoptical axis deflection speed instruction signal v(p An optical axisdeflection speed detector 39 detects if the optical axis is beingdeflected or not based on the presence or absence of the optical axisdeflection speed instruction signal v_(lp). In case that the opticalaxis is being deflected, the switching circuit 37 is turned on.

The operation will now be described.

The laser beam generated by the laser source 1 is applied to thegalvanometer mirrors 5X and 5Y via the modulator 2, mirror 3, andexpandor 5. The angles (positions) of the galvanometer mirrors 5X and 5Yare controlled by the writing control circuit 7. The X axis mirror 54Xis driven as in the following. The position control circuit 31X outputsa speed instruction signal v_(xp) proportional to the deviation betweenthe X axis position instruction signal P_(xp) supplied from thecontroller 8 and the X axis position instruction signal P_(xf) detectedby the position detector 53X. The speed instruction signal v_(xp), isinputted to the speed control circuit 33X via the speed limit circuit32X. The speed control circuit 33X outputs a current instruction signali_(xp) proportional to the speed deviation between the speed instructionsignal v_(xp) and the speed detecting signal v_(xf), and applies it tothe current control circuit 34X. The current control circuit 34X isinputted with the current instruction signal i_(xp) and the currentdetecting signal i_(xf) fed back from the mirror driver circuit 51X, anddrives the mirror 54X under control of the mirror driver circuit 51X.With the above operations, the galvanometer mirror 5X of the X axis iscontrolled to coincide with the X axis position instruction signalP_(xp). Similarly to the galvanometer mirror 5X of the X axis, thegalvanometer mirror 5Y of the Y axis is driven to the positioncoinciding with the Y axis position instruction signal P_(yp). The laserbeam whose position on the liquid crystal 10 was determined by thegalvanometer mirrors 5X and 5Y, is collected by the scan lens 6 to beapplied on the liquid crystal at the determined position. Writing to theliquid crystal 10 is attained by continuously performing the aboveoperations.

The image written on the liquid crystal is projected on the screen 16 bymeans of the projection system 15. The operation of the projectionsystem 15 is well known so that the detailed description therefor isomitted. To erase the entire image written on the liquid crystal 10, ahigh AC voltage (e.g., 100 volt) from the controller 8 is appliedbetween an aluminum electrode and a transparent electrode of the liquidcrystal 10. Alternatively, to selectively erase the image, a low voltage(e.g., about 20 volt) is applied between both electrodes of the liquidcrystal 10, and a laser beam generated by the laser source 1 is appliedto the area to be erased.

The writing to the liquid crystal 10 and projection display thereof areperformed as described above. Assume that the laser beam is scanned fromposition P0 to position P3 on the liquid crystal 10 for writing theretoas shown in FIG. 4(a). In this case, the galvanometer mirrors 5X and 5Yare deflected at a speed as shown in FIG. 4(b). Starting from positionP0 at time t₀, the galvanometer mirrors are driven under constantacceleration to reach position Pl at time t₁ when a 100% speed isattained. From time t₁ to time t₂ at position P2, the galvanometermirrors move at a constant speed (100% speed). From position P2, theyare reduced in speed under constant deceleration to stop at position P3at time t₃.

The optical axis deflection speed calculating circuit 35 inputted withthe X and Y axis speed detecting signals v_(xf) and v_(yf) obtains anoptical axis deflection speed v_(lo) in accordance with the followingequation: ##EQU1##

The optical axis deflection speed v_(lo) obtained by the optical axisdeflection speed calculating circuit 35 is inputted to the speed/lightintensity conversion circuit 36, which then outputs an optical axisdeflection speed v_(lo) and a light intensity control signal l_(c)having a characteristic as shown in FIG. 6. The speed/light intensityconversion circuit 36 may be constructed of a function generator,read-only memory or the like. The optical axis deflection speedinstruction calculating circuit 38 inputted with the speed instructionsignals v_(xp) and v_(yp) of the X and Y axes obtains an optical axisdeflection instruction signal v_(lp) in accordance with a similarequation to the equation (1). An optical axis deflection detectingcircuit 39 turns of the switching circuit 37 when an optical axisdeflection speed instruction signal v_(lp) is present. While writing bymoving the optical axis from position P0 to position P3 as shown in FIG.4(a). The optical axis deflection speed instruction signal v_(lp) ispresent. Thus the switching circuit 37 remains turned on only while theoptical axis moves. Upon turning-on of the switching circuit 37, thelight intensity control signal l_(c) generated by the speed/lightintensity conversion circuit 37 is applied to the modulator 2. The lightintensity control signal l_(c) becomes larger as the optical axisdeflection speed v_(lo) becomes higher, as shown in the performancegraph of FIG. 6. The piezoelectric element driver circuit 26constituting the modulator 2 makes the piezoelectric element 23 vibratein proportion to the magnitude of the light intensity control signall_(c). The piezoelectric element applies ultrasonic waves to the glassmember 22 which, in turn, generates therein compressional waves by whichthe refractive index of the laser beam changes. The laser beam I_(i)collected by the lens 24 is split into a first order beam I₀ and asecond order beam I₁ by means of the diffraction grating 22 realized bythe compressional waves in the glass member 22. The first order beam I₀is made parallel by the lens 24 and applied to the liquid crystal 10,while the O order beam I₀ is intercepted by the shutter 25. Thepiezoelectric element 23 generates ultrasonic waves whose intensity isproportional to the magnitude of the light intensity control signall_(c). As the ultrasonic intensity becomes higher, the diffractionefficiency becomes good, to thereby make the intensity of the firstorder beam I₁ high. The modulator 2 changes the intensity of the laserbeam to be applied to the liquid crystal in the above-described manner.Consider now that during the time of acceleration from position P0 toposition P1, the laser beam intensity becomes high as the optical axisdeflection speed v_(lo) becomes high, as shown in FIG. 5. Writing iscarried out as described above.

As seen from the above description, according to the present invention,the intensity of the laser beam applied to the liquid crystal 10 is madehigh as the optical axis deflection speed v_(lo) becomes high duringacceleration of the deflection speed of the galvanometer mirrors 5X and5Y, i.e., the axis deflection speed. Contrary, during deceleration, theintensity of the laser beam applied to the liquid crystal 10 is made lowas the optical axis deflection speed v_(lo) becomes low. Therelationship between the optical axis deflection speed v_(lo) and thewritten line width becomes as shown in FIG. 7 on condition that theintensity of the laser beam applied to the liquid crystal 10 ismaintained constant. As seen from FIG. 7, as the optical axis deflectionspeed v_(lo) becomes low, the written line width becomes broaderrelative to that at 100% of the optical axis deflection speed v_(lo).Alternatively, the relationship between the laser beam intensity and thewritten line width becomes as shown in FIG. 8, which shows theexperimental results by the inventors, on condition that the opticalaxis deflection speed v(o on the liquid crystal 10 is maintainedconstant. As apparent from FIG. 8, the laser beam intensity and thewritten line width is proportional to each other. It is noted that alaser beam intensity less than 10% is impossible for use in writing.

As apparent from the performances shown in FIGS. 7 and 8, therelationship between the optical axis deflection speed and the laserbeam intensity becomes as shown in FIG. 9. Accordingly, it is possibleto maintain the written line width constant by controlling the laserbeam intensity l_(c) against the optical axis deflection speed v_(lo) asshown in FIG. 6.

FIG. 10 shows the written line width from position P0 to position P1 ofFIG. 4 under acceleration. The solid line shown in FIG. 10 correspondsto the line width obtained according to the present invention, and thedot line corresponds to the line width where the laser beam intensity isnot varied with the optical axis deflection speed. As apparent from FIG.10, according to the present invention, it is possible to maintain thewritten line width constant irrespective of the high or low optical axisdeflection speed.

The foregoing description has been directed to writing to the liquidcrystal 10. However, it is also possible to apply to the case where theselective erase with a low voltage applied between the electrodes of theliquid crystal is performed, thus maintaining a constant erase linewidth. Accordingly, the erasure can be precisely made and the visibilityis improved as a matter of course.

Next, the speed limit values v_(xL) and v_(yL) supplied to the speedlimit circuits 32X and 32Y will be explained.

The speed limit values may be identical to a constant speed (100% speed)v_(max). v_(max) is determined in consideration of the mechanicaldeflection speeds of the galvanometer mirrors 5X and 5Y and theintensity of the laser beam generated by the laser source 1. In casewhere the response time in speed of the writing control system differbetween the X and Y axes, it is preferable to make the speed limitvalues v_(xL) and v_(yL) variable as in the following. For example,consider that a straight line is written from point A (x₀, y₀) to point(x₇, y₃) on the X-Y coordinate plane of the liquid crystal 10 as shownin FIG. 11. The writing control circuit 7 supplies the positioninstruction signals P_(xp) and P_(yp) of the X and Y axes representingthe ultimate coordinate (x₇, y₃) and simultaneously therewith,determines the limit values v_(xL) and v_(yL) on the basis of thefollowing concept.

The limit values v_(xL) and v_(yL) on the basis of the followingconcept. ##EQU2## From the equations (2) and (3), the X axis speed limitvalue v_(xL) and the Y axis speed limit value v_(yL) are given by thefollowing equations. ##EQU3## For writing from point A to point B ofFIG. 11, the limit values v_(xL) and v_(yL) become as shown in FIG. 12,the former being larger than the latter. By changing the limit valuessufficing the equations (4) and (5) every time writing is performed, itis possible to correctly follow the locus of the straight line even ifthe response time in speed of the writing control system differs betweenthe X and Y axes. In case a curved line as desired is to be written, thelimit values v_(xL) and v_(yL), are continuously changed using theparameter of angle θ.

As described so far, according to the present invention, a constantwritten line width is ensured merely by changing the intensity of thelaser beam applied to the liquid crystal in accordance with the opticalaxis deflection speed. Therefore, thermal writing of a line with aconstant width can be performed in short time.

In the above embodiment, the intensity of the laser beam applied to theliquid crystal is adjusted using the modulator. If a semiconductor lasersource is used as the laser source, the semiconductor laser source canvary its laser beam intensity by itself. Obviously, the presentinvention is also applicable to semiconductor lasers. Further, it isapparent that the modulator may be of the type utilizing theelectrooptical effect. Furthermore, the similar advantageous effects canbe enjoyed by embodying the present invention using three liquidcrystals for color display. In addition, it is apparent that the writingcontrol circuit, screen controller and the like may be substituted for acomputer to digitally perform such functions.

We claim:
 1. A liquid crystal projection display system comprising:(a)means for generating a laser beam; (b) a liquid crystal to which thermalwriting or erasing is effected with said laser beam irradiated from saidlaser beam generating means; (c) means for scanning said laser beamacross said liquid crystal; and (d) laser intensity varying meansdisposed between said laser beam generating means and liquid crystal forincreasing the intensity of said laser beam in accordance with theincrease of the scanning speed so as to maintain the energy of a laserbeam spot on said liquid crystal constant during a full scanning periodincluding acceleration period, constant speed period and decelerationperiod; thereby to obtain a constant width line thereon.
 2. A liquidcrystal projection display system according to claim 1, wherein saidlaser beam scanning means is a galvanometer type deflector having twogalvanometer mirrors for an X-axis and a Y-axis perpendicular with eachother for irradiating said laser beam to said liquid crystal and forscanning said laser beam across said liquid crystal by means ofmechanical displacement of said galvanometer mirrors.
 3. A liquidcrystal projection display system comprising:(a) a semiconductor lasersource for generating a laser beam and capable of adjusting theintensity of said laser beam; (b) a liquid crystal to which thermalwriting or erasing is effected with said laser beam irradiated from saidsemiconductor laser source; (c) means for scanning said laser beamacross said liquid crystal; and (d) means for controlling saidsemiconductor laser source and increasing the intensity of said laserbeam generated by said semicondutor laser source in accordance with theincrease of the scanning speed so as to maintain the energy of a laserbeam spot on said liquid crystal constant during a full scanning periodincluding acceleration period, constant speed period and decelerationperiod; thereby to obtain a constant width line thereon.
 4. A liquidcrystal projection display system according to claim 3, wherein saidlaser beam scanning means is a galvanometer type deflector having twogalvanometer mirrors for an X-axis and a Y-axis perpendicular with eachother for irradiating said laser beam to said liquid crystal and forscanning said laser beam across said liquid crystal by means ofmechanical displacement of said galvanometer mirrors.
 5. A liquidcrystal projection display system comprising:(a) means for generating alaser beam; (b) a liquid crystal to which thermal writing or erasing iseffected with said laser beam irradiated from said laser beam generatingmeans; (c) a galvanometer type deflector having two galvanometer mirrorsfor an X-axis and a Y-axis perpendicular with each other for irradiatingsaid laser beam to said liquid crystal and for scanning said laser beamacross said liquid crystal by means of mechanical displacement of saidgalvanometer mirrors; (d) writing and erasing control means forsupplying control signal to said galvanometer type deflector and fordisplacing mechanically said two galvanometer mirrors; (e) positioncontrol means for outputting a speed instruction signal corresponding toa deviation between a position instruction signal from said writing anderasing control means and a position detecting signal; (f) speed controlmeans for outputting a current instruction signal corresponding to adeviation between said speed instruction signal and a scanning speeddetecting signal of said galvanometer type deflector; (g) currentcontrol means for controlling a current to be supplied to a drive sourcefor driving said galvanometer type deflector in accordance with saidcurrent instruction signal; (h) speed limit means provided between saidposition control means and said speed control means for limiting themagnitude of said speed instruction signal; and (i) laser intensityvarying means disposed between said laser beam generating means andliquid crystal for increasing the intensity of said laser beam inaccordance with the increase of the scanning speed so as to maintain theenergy of a laser beam spot on said liquid crystal constant during afull scanning period including acceleration period, constant speedperiod and deceleration period; thereby to obtain a constant width linethereon.
 6. A liquid crystal projection display system according toclaim 5, wherein said speed limit means is set with a constant speedlimit value.
 7. A liquid crystal projection display systemcomprising:(a) means for generating a laser beam; (b) a liquid crystalto which thermal writing or erasing is effected with said laser beamirradiated from said laser beam generating means; (c) a galvanometertype deflector having two galvanometer mirrors for an X-axis and aY-axis perpendicular with each other for irradiating said laser beam tosaid liquid crystal and for scanning said laser beam across said liquidcrystal by means of mechanical displacement of said galvanometermirrors; (d) writing and erasing control means for supplying controlsignal to said galvanometer type deflector; (e) position control meansfor outputting a speed instruction signal corresponding to a deviationbetween a position instruction signal from said writing and erasingcontrol means and a position detecting signal; (f) speed control meansfor outputting a current instruction signal corresponding to a deviationbetween said speed instruction signal and a scanning speed detectingsignal of said galvanometer type deflector; (g) current control meansfor controlling a current to be supplied to a drive source for drivingsaid galvanometer type deflector in accordance with said currentinstruction signal; and (h) speed limit means provided between saidposition control means and said speed control means for limiting themagnitude of said speed instruction signal.